[p4] How can this happen?

[Tetrick, Cary]

We had a situation where one user checked out a file that was type
binary+l
While he had it checked out, another user apparently made changes to the
same file and submitted it, forcing the first user to have to resolve
the file.

Can anyone explain how this can happen? Shouldn't the second user have
been unable to check out the file because it was locked?